Foundry cores and molds used in making metal castings are normally prepared from a composition including sand or similar material and a curable or polymerizable binder coated on the sand particles. The purpose of this binder coating is to permit the mixture to be hardened after it is first shaped or molded into a desired form. Typically, after the aggregate material and binder have been mixed, the resulting mixture is rammed, blown or otherwise formed to the desired shape or pattern and then cured with the use of catalysts and/or heat to a solid, cured state.
When molten metal is poured onto a sand mold, it solidifies taking the shape or pattern of the mold. The temperature of the molten metal is so high that the resin binder burns out of the mold. The mold then collapses leaving free-flowing sand that can be reused to make a new mold.
Different processes for forming molds and cures have been developed in the foundry industry. One process known as the "hot-box" process requires that the mixture of aggregate material and binder be cured and hardened in a holding pattern or core box while subjected to heat. Other processes which use binder systems that do not require heating in order to bring about curing or hardening are known as "no-bake" processes. The present invention is directed to an improved cold-setting binder composition for use in a "no-bake" process.
Phenol-formaldehyde resole resins, as well as furfuryl alcohol, urea-formaldehyde resins, and their mixtures have been used as binders in "no-bake" processes. In such processes, the foundry sand is usually mixed with an aromatic sulfonic acid catalyst before the resin binder is added to the mixture.
In order to increase the cure speed in "no-bake" systems, it is frequently necessary to increase the amount of sulfonic acid catalysts employed. This is particularly true in cold weather when the sand is at a low temperature. Use of higher catalyst levels is undesirable not only from the cost basis but because such sulfonic acid catalyst levels cause higher sulfur dioxide emissions during pouring of the molten metal. Furthermore, when sand with high catalyst levels is reclaimed for reuse, it has high residual acidity and higher amounts of elemental sulfur. Such residual acidity means larger amounts of new sand must be added to the reused sand. The elemental sulfur is also undesirable since it can be picked up by the molten metal, resulting in serious casting defects.
One way to increase the curing speed of a phenolic resole "no-bake" resin binder is disclosed in U.S. Pat. No. 4,336,179. The resins of that disclosure contain a monohydric phenol which is added after the resole is formed. However, these products often require the use of a fairly high level of catalyst.
We now have discovered that when certain polyhydroxy compounds are added to cold-setting "no-bake" compositions, they accelerate curing of these resins without the need to use increased amounts of catalyst. Such accelerators permit the use of even less than normal amounts of catalyst thereby reducing the level of sulfur dioxide emission, the residual acidity of the reused sand, and the amount of undesirable elemental sulfur remaining in the reclaimed sand.